Dryer section of a paper machine

Abstract: Dryer section of a paper machine, comprising so-called normal drying groups (R N ) provided with single-wire draw (12), in which the heated contact-drying cylinders (10) are placed in the upper row and the leading cylinders or rolls (11) in the lower row. There is/are a drying module (20-23) or modules (20A,20B-23) between and/or inside the normal groups (R N ), in which modules the opposite side of the web (W), in relation to the side placed against said contact-drying cylinders (10), is placed against the heated cylinder face (21). The drying module comprises a single drying cylinder (20) or cylinders (20A,20B), against whose heated cylinder face (21) the web (W) is placed in direct contact. The drying module (20;20A,20A,20B,23) comprises a drying-wire loop (22) of its own, which guides the web (W) and presses its opposite side, in relation to the web face placed against the drying cylinders (10) in the normal groups (R N ), against the heated face(s) (21) of said single cylinder (20;20A) or cylinders (20A, 20B) over a sector a, whose magnitude has been dimensioned as a > 180°. The diameter (D₂) or the diameters of the single drying cylinder (20;20A) or cylinders (20A,20B) is/are dimensioned as substantially larger than the diameter (D₀) of the contact-drying cylinders (10) in the normal groups (R N ).

Description: The invention concerns a dryer section of a paper machine, comprising so-called normal drying groups provided with single-wire draw, in which the heated contact-drying cylinders are placed in the upper row and the leading cylinders or rolls in the lower row, and in which dryer section there is/are a drying module or modules between and/or inside said normal groups, in which modules the opposite side of the web, in relation to the side placed against said contact-drying cylinders, is placed against the heated cylinder face. The highest web speeds in paper machines are currently already of an order of 25 metres per second, but, before long, the speed range of 25...40 m/s is also likely to be taken into use. With the current highest running speeds and with the still higher future running speeds, especially the dryer section has become and will be the bottleneck of the runnability of a paper machine. The quality requirements imposed on the paper produced, in particular on fine and copying paper, are even now quite strict, and will become ever stricter. Particularly high requirements are imposed on the symmetry of the paper in the z-direction and on the properties of the face at both sides as well as on the stability of the paper structure as the paper is heated rapidly in a copying or printing process. These requirements of quality imposed on a paper product impose particularly high requirements on the dryer section of a paper machine, which requirements are ever more difficult to meet with increasing running speeds. As is known in prior art, twin-wire draw and/or single-wire draw is/are used in multi-cylinder dryers of paper machines. In the former case, the groups of drying cylinders have two wires, which press the web, one from above and the other one from below, against the heated cylinder faces. Between the rows of cylinders, which are usually horizontal rows, the web has free and unsupported draws, which are susceptible of fluttering, which may cause web breaks. This is why, in recent years, increasing use has been made of said single-wire draw, in which, in each group of drying cylinders, there is only one drying wire, on whose support the web runs through the whole group so that the drying wire presses the web against the heated cylinder faces on the drying cylinders, whereas the web remains at the side of the outside curve on the leading cylinders between the drying cylinders. Thus, in single-wire draw, the drying cylinders are placed outside the wire loop and the leading cylinders inside said loop. In the prior-art normal groups with single-wire draw, the heated drying cylinders are placed in the upper row and the leading cylinders in the lower row, said rows being, as a rule, horizontal and parallel to one another. In the applicant's FI Patent No. 54,627 (corresp. US Pat. 4,202,113) , it is suggested that the above normal single-wire groups and so-called inverted single-wire groups be placed one after the other, in which inverted groups the heated drying cylinders are placed in the lower row and the leading suction cylinders or rolls in the upper row, the principal objective being to dry the web symmetrically from both of its sides. Messrs. Beloit Corp. have also made certain suggestions concerning a dryer section that comprises normal and inverted cylinder groups, in which respect reference is made to the international published patent applications WO 88/06204 and WO 88/06205 . With respect to the prior art, reference is made further to the US Patent No. 2,537,129, in whose Fig. 4 an inverted cylinder group is shown, which is followed by a drying module consisting of a single Yankee cylinder. In the following, when the terms "normal (drying) group" and "inverted (drying) group" are used, what is meant is expressly the cylinder groups with single-wire draw of the type mentioned above. With the use of single-wire draw in the area of the whole dryer section, various problems have occurred, for which the present invention is supposed to suggest novel, efficient solutions. These problems include the large length of the dryer section, which increases the costs of the dryer section and of the machine hall. Problems have also arisen from the difference in speed between the paper web and the wires, which has resulted in wear of the wires and, at its worst, even in a paper break in the dryer section. The use of an inverted group has also resulted in problems in the removal of broke, which has resulted in increased break times and in lowered efficiency. As a rule, said problems tend to become worse when the running speeds of paper machines become higher. The general object of the present invention is to provide novel solutions for the problems discussed above. It is a further object of the present invention to permit wider possibilities of regulation and more accurate control of the drying process. It is an additional object of the invention to provide a dryer section by whose means a web can be produced which is sufficiently symmetric in the z-direction and which also possesses the surface properties required by the purpose of use of the paper at both sides. In view of achieving the objectives stated above and those that will come out later, the invention is mainly characterized in that said drying module comprises a single drying cylinder or cylinders, against whose said heated cylinder face the web is placed in direct contact, and that said drying module comprises a drying-wire loop of its own, which guides the web and presses its opposite side, in relation to the web face placed against the drying cylinders in the normal groups, against the heated face(s) of said single cylinder or cylinders over a sector a, whose magnitude has been dimensioned as a > 180°. Drying modules in accordance with the invention may be placed in a gap or gaps between normal groups and/or, in particular cases, inside a normal group or groups. As a rule, the drying modules preferably consist of one single large-diameter drying cylinder, but in exceptional cases, especially when the drying module is placed inside a normal group, it is possible to employ more than one, preferably two, successive drying cylinders, which have a preferably common loop of drying wire. In a preferred embodiment of the invention, the prior-art inverted groups of drying cylinders have been replaced by a drying module, which comprises a single drying cylinder, whose diameter is larger than the diameters of normal drying cylinders. In this way, a more efficient dryer section is provided, whose susceptibility of breaks is lower, as compared with the use of inverted multi-cylinder groups. Moreover, in the event of a break, the standstill times become shorter, because the time-consuming cleaning of an inverted multi-cylinder group is substantially omitted or becomes substantially quicker. In this embodiment, single large cylinders can be placed, as required, in one or, preferably, in several group gaps, so that sufficiently symmetric drying of the paper web from both sides can be accomplished. According to the invention, a dryer section of reduced length can be provided, whereby substantial economies can be obtained in the investment costs of the dryer section as well as in the costs of the machine hall. In the invention, the transfers from a normal group to a single large cylinder and from said cylinder to a normal group can be carried out as a closed and/or open draw depending on what an optimal operation of the drying process and elimination of breaks require from the arrangement of the various parts and from the geometry of equipment. In the invention, as one parameter of regulation, it is possible to use the steam pressure in the single large cylinders and the temperature of their cylinder faces, in particular so that said temperature is chosen higher than the cylinder temperatures in the normal groups, whereby the proportion of the large cylinder in the drying of the web can be kept at a sufficient level. The proportion of a large cylinder in the drying of the web can also be affected by means of the tightness of the drying wires most advantageously so that, in a drying module that comprises a large cylinder, a higher wire tightness is used than in so-called normal groups. In the invention, drying modules that comprises a single drying cylinder or, inside a normal group, in exceptional cases, drying modules that comprise several drying cylinders, can be placed exactly at the locations at which it is preferable in view of the whole of the drying process. Moreover, when the invention is applied in practice, the prior-art normal groups can be modified so that, in them, a reduced number of drying and leading cylinders and/or a smaller cylinder diameter is/are employed, so that the proportion of the drying of the web carried out by means of said drying modules can be brought to a sufficiently high level. As a rule, in normal groups, the contact cylinders and the leading cylinders are placed in the same horizontal planes as compared with one another. However, it should be emphasized that the invention can also be applied to dryer sections in which the principal directions of the normal groups are inclined upwards or downwards or even vertical. Examples of such embodiments are shown in Figs. 8, 9 and 10 in the drawing in the present application. In the following, the invention will be described in detail with reference to some exemplifying embodiments of the invention illustrated in the figures in the accompanying drawing, the invention being by no means strictly confined to the details of said embodiments. Figure 1 is a schematic side view of a dryer section in which there are several successive normal groups and, between them, single large cylinders fitted in accordance with the invention. Figure 2 is an enlarged view of the components and group-gap draws at the proximity of the first large cylinder as shown in Fig. 1. Figure 3 shows a second embodiment of the invention in a way similar to Fig. 1. Figure 4 shows a third embodiment of the invention in a way similar to Figs. 1 & 3. Figure 5 illustrates axial sectional views of the mantle of a leading cylinder at the planes V-V denoted in Fig. 2. Figure 6 shows an embodiment of the invention in which a drying module that comprises a single large cylinder is placed inside a normal group. Figure 7 shows such a modification of the embodiment shown in Fig. 6 in which a drying module fitted inside a normal group comprises two large cylinders placed inside the same wire loop. Figure 8 shows an embodiment of the invention in which a drying module that comprises a single large cylinder is placed between two successive normal groups, of which groups the former one has a rear half whose principal direction is inclined downwards, whereas the latter one has a forward half that is inclined upwards. Figure 9 shows such a modification of the embodiment shown in Fig. 8 in which the rear half of the former normal group has a principal direction that is inclined upwards, whereas the forward half of the latter normal group has a principal direction that is inclined downwards. Figure 10 shows an embodiment in which a drying module in accordance with the invention is placed inside a normal group which has a forward part that is inclined downwards and a rear part that is inclined upwards. In the figures, the steam-heated drying cylinders are denoted with the reference numeral 10, and the leading cylinders with the reference numeral 11. The normal groups R N include an upper drying wire 12, which is guided by the guide rolls 13,13a. In the figures, the frame construction 18 of the dryer section is illustrated schematically. In the normal groups R N the free lower faces of the cylinders 10 are provided with doctors 16, and in the pockets above the leading cylinders 11 there are blow boxes 14, and in the intermediate spaces below the drying cylinders 10 there are blow boxes 15, by whose means the runnability is promoted and the evaporation is improved, above all by intensifying the ventilation of the intermediate spaces between the cylinders 10 and 11 and by reducing the differences in pressure induced in the various nips. On the large cylinders 20 in accordance with the invention, against the smooth cylinder face heated to the temperature t 1 , the opposite side of the web W is placed in contact, opposite in relation to the web face that is in contact with the cylinders 10 in the normal groups R N . In this way, sufficiently symmetric drying of the web W from both sides can be accomplished. According to Figs. 1 to 4, after the press section (not shown), there are two normal groups R N1 and R N2 in the dryer section, between which groups the paper web W to be dried has a closed draw. The number of said groups may, of course, also be higher. According to the invention, between the second and the third normal group R N2 , R N3 , a single large cylinder 20 is fitted, on which there is the drying wire 22. This wire 22 presses the web W to be dried against the heated smooth cylinder face 21 of the large cylinder 20 over the sector a. In Figs. 1 to 4, the diameter D 2 of the large cylinders 20 is substantially larger than the diameter D 0 of the steam-heated drying cylinders 10 in the normal groups R N . In particular cases, the diameter D 2 of the large cylinders 20 can also be dimensioned as substantially equal to, and in entirely special exceptional cases also somewhat smaller than, the diameter D 0 of the drying cylinders 10. In different drying modules it is also possible to employ unequal diameters D 2 of the large cylinders 20, compared with one another. The most important parameters of the geometry of the drying module formed by a large cylinder 20 and the other advantageous constructional features will be described in more detail later in particular with reference to Fig. 2. As regards its construction, a large cylinder 20 is preferably a cylinder similar to a conventional drying cylinder, and for its manufacture it is possible to employ a technology substantially equal to that used for the manufacture of prior-art drying cylinders or corresponding Yankee cylinders. As to its construction, a large cylinder 20 may also be a steel cylinder made by welding out of pieces of metal plate. The tightening tension T of the loops 22 of the drying wire of the large cylinders 20 can be chosen in accordance with the particular requirements of the drying carried out by means of the large cylinder 20, preferably so that said tension T is higher than the corresponding tension of the drying wires 12 in the normal groups R N . According to Fig. 1, after the second normal group R N2 , the web W is passed on the drying wire 12 to the guide roll 13a, after which the web has a short open draw W 0 , whereupon the web W is transferred, at the guide roll 23a, onto the drying wire 22 of the large cylinder 20 and, being pressed by said drying wire 22, over the sector a, into direct contact with the heated cylinder face 21 of the large cylinder 20. Here-upon the web W is separated from the cylinder face 21 and is transferred after the guide roll 23a as a short open draw W 0 , at the guide roll 13a, onto the drying wire 12 of the subsequent normal group R N3 . A corresponding single large cylinder 20 and web-transfer arrangement are provided between the normal groups R N3 and R N4 . The dryer section illustrated in Fig. 1 and in the following figures can be modified within the scope of the invention in many different ways. One quite usable modification of Fig. 1 is such that the guide rolls 13a and 23a are placed so that the open draws W 0 have been replaced by a closed draw of the web and/or that said guide rolls 13a and 23a are rolls with no suction zone, even smooth-faced rolls provided with a solid mantle. According to Figs. 1 and 2, the horizontal distance S 2 between the adjacent drying cylinders 10 in successive groups R N2 -R N3 , R N3 -R N4 ... is dimensioned as substantially equal to the horizontal distance between adjacent drying cylinders 10 inside the normal groups R N . Said distance S 2 is typically in a range of S 2 ≈ 150 mm ... 500 mm. In this way, it is possible to provide a very compact dryer section, which is substantially shorter as compared with the use of inverted multi-cylinder groups. Fig. 3 shows an embodiment of the invention in which said horizontal distance S 23 is substantially larger than S 2 , typically S 23 ≈ (2...3) x S 2 . In such a case, the dryer section becomes somewhat longer, but, at the same time, more space becomes available for the guide rolls 13a,23a and, if necessary, also for blow boxes or equivalent, which are represented by the blow box 14A in Fig. 3. Fig. 4 illustrates an embodiment of the invention in which the horizontal distance S 24 between adjacent cylinders 10 in successive groups R N at the large cylinder 20 is substantially larger than in Figs. 1 to 3, preferably S 24 ≈ (3...8) x S 2 . In such a case, in connection with the transfer in connection with the group gap, as the guide rolls 13A and 23A of the wires 12 and 22, it is possible to use guide rolls and/or suction rolls whose diameter (D 4 ) is larger than that of normal guide rolls 13,23 and which are provided, e.g., with internal suction boxes and with appropriately fitted suction zones, which ensure undisturbed transfer of the web W at the group gaps even at high web speeds. As the guide rolls 13A and 23A, it is also possible to use leading cylinders marketed by the applicant under the trade mark "Vac-Roll". According to Fig. 4, the draws W 1 and W 2 at the group gaps are fully closed, so that the web W is transferred from the preceding wire 12 onto the wire 22 of the single large cylinder 20 as a fully closed draw W 1 . A corresponding closed draw W 2 is provided at the outlet side of the wire 22 onto the wire 12 of the group R N3 . The diameter D 4 of the guide rolls 13A,23A is typically in a range of D 4 ≈ 600 mm ... 1500 mm. The construction as shown in Fig. 4 is also advantageous in the respect that, owing to the relatively wide open space S 24 , the area of the single large cylinder 20 is open upwards, so that removal of broke through said open space S 24 is rapid and that the transfer of the web at the group gaps is highly reliable and undisturbed. Above and in the following description, for the sake of conciseness, the designation leading cylinder 11 has been used, which, with some exceptions, refers to suction cylinders or rolls against which the drying wire 12 enters into direct contact while the web W is placed at the side of the outside curve. Thus, the leading cylinders 11 are placed inside the drying-wire loops 12, and the drying cylinders 10 outside. Even though, in the following, the designation leading cylinder 11 will be used, in some cases these can also be replaced by rolls of smaller diameter, such as leading suction rolls, which are provided with an internal suction box. With some exceptions, the leading cylinders 11 are in the present invention preferably leading cylinders marketed by the applicant under the trade mark VAC-Roll (diameter D 2 ≈ 1500 mm), whose mantle 11V, which is provided with a grooved outside face 11', is perforated and whose interior communicates with negative pressure p 0 , the web W being held on the turning sectors of the leading cylinders 11 by means of the difference in pressure produced by means of said negative pressure. Fig. 5 shows axial sectional views of the mantle 11V of the leading suction roll 11 taken along the plane V-V in Fig. 2. The grooved face 11' in said mantle 11V consists of annular grooves 11R passing around the mantle, the depth of said grooves being denoted with r 0 and the width of the groove with l 0 , and the width of the mantle portions of full wall thickness between the grooves with l 1 . The perforations 11P that pass through the mantle 11V are opened into the groove 11R bottoms. The diameter of the holes is denoted with , and the full thickness of the mantle 11V with r 1 . In the following, a preferred example of dimensioning of a grooved mantle as shown in Fig. 5 will be given: r 0 ≈ 4 mm, l 0 ≈ 5 mm, r 1 ≈ 30 mm, l 1 ≈ 16 mm,  ≈ 4 mm. The spacing and the diameters  of the perforations 11P are chosen preferably so that the percentage of the holes in the total area of the groove 11R bottoms is about 0.5...2 %. The negative pressure p 0 is preferably in a range of p 0 ≈ 1 kPa ... 5 kPa. With respect to the other constructional details of said VAC-Rolls, reference is made to the applicant's FI Patent No. 83,680 (corresp. US Pat. 5,172,491) . In the following, with reference to Fig. 2, the most important parameters of construction and dimensioning of a drying module consisting of a large cylinder as shown in Figs. 1 to 4 and preferred exemplifying embodiments of same will be described. As was stated above, the diameter D 2 of a single large cylinder 20 should preferably be dimensioned substantially larger than the diameter D 0 of the drying cylinders 10, preferably so that D 2 ≈ (1.1...1.7) x D 0 . Typically, D 0 ≈ 1800 mm and D 1 ≈ 1500 mm, in which case D 2 is preferably dimensioned in a range of D 2 ≈ 2000 mm ... 3000 mm. The diameter D 3 of the wire 12,22 guide rolls 13 is typically D 3 ≈ 500 mm ... 800 mm. In Figs. 1 to 6, the cylinders 10 in the normal groups R N are placed in the same horizontal plane T 0 -T 0 in all groups, and so are the leading cylinders 11 placed in the same horizontal plane T 1 -T 1 with each other. The difference in height H 0 between said horizontal planes T 0 and T 1 is typically H 0 ≈ 900 mm ... 1800 mm. According to Fig. 2, the centres K of the single large cylinders 20 in accordance with the invention are placed considerably below the plane T 1 -T 1 , in the plane T 2 -T 2 . With the dimensioning of the cylinders 10,11 and 20 given above, the difference in height H 1 between the planes T 1 and T 2 is typically H 1 ≈ 0...1500 mm, mainly depending on the diameter of the large cylinder 20 and on the web-draw geometry to be used. As was stated above, the shortest horizontal distance S 2 between adjacent cylinders 10 is preferably substantially equal to the distance between the cylinders 10 in the normal groups R N . The guide rolls 13A and 23A of the wires 12 and 22 are preferably placed in substantially the same horizontal plane or at a slight relative difference in height, depending on what is required by an optimal transfer of the web at group gaps. Besides by the cylinder diameter D 2 , the evaporation capacity of the single large cylinders 20 is also affected by the magnitude of the covering sector a of the web W. Said sector a is, as a rule, chosen in the range of a ≈ 180° ... 300°, preferably in the range of a ≈ 220° ... 270°. Moreover, the drying capacity of the large cylinders 20 can be influenced by means of the temperature t 1 of its face 21. Said temperature t 1 is preferably arranged somewhat higher than the corresponding surface temperature of the cylinders 10, which is achieved, for example, by in the large cylinders 20 using a higher steam pressure than in the cylinders 10 in the normal groups R N . The drying process and the transverse shrinkage of the web can, on the large cylinder 20, also be affected to some extent by means of the wire 22 structure, in particular by its permeability and its tension T. Said tension T is preferably chosen as somewhat higher than the tension of the wires 12, typically in a range of T ≈ 2 kN/m ... 5 kN/m. The construction shown in Fig. 2 is preferably such that it is symmetric in relation to a vertical plane transverse to the machine direction and placed through the centre K of the single large cylinder 20. In the embodiments of the invention shown in Figs. 1 to 4, the drying modules consisting of single drying cylinders 20 of large diameter can be placed over the length of the dryer section exactly at the points at which it is most advantageous in view of the whole of the drying process. Typically, in a fine-paper or newsprint machine, there are 6...9 normal groups R N and 2...3 drying modules in accordance with the invention, said drying modules being placed mainly in the gaps between the normal groups in the rear end of the dryer section. If the proportion of the evaporation taking place from the different faces of the web W by means of the drying modules in accordance with the invention cannot be controlled to a sufficient extent by the means described above, i.e. by means of the dimensioning of the diameter D 2 and the covering sector a of the single cylinders 20, the wire 22 tension, and/or by the choice of the temperature t 1 of the cylinder 20 faces, said drying proportion may be increased further by using normal groups R N shorter than normal, i.e. by in the normal groups providing, e.g., just 2...4 drying cylinders, while their usual number is 5...6. In stead of, or in addition to, said means, it is possible, in the normal groups R N , to use a smaller diameter D 0 of a drying cylinder than what was stated above, for example by choosing D 0 in the range of D 0 ≈ 1500...1750 mm. In Fig. 6, an embodiment of the invention is shown in which there is no drying module, but a closed draw, between the successive normal groups R N1 , R N2 and R N31 . In stead, inside the latter normal group R N31 , a drying module in accordance with the invention is arranged, which comprises a large cylinder 20A, against whose heated face 21 the opposite face of the web W is placed, opposite in relation to the web face placed in contact with the drying cylinders 10 in the normal groups R N . From the drying cylinder 10a of the normal group R N31 , the web W arrives as a short open draw W 0 onto the large cylinder 20A and is, in a corresponding way, transferred from the large cylinder 20A as a short open draw W 0 onto the drying wire 12 of the group R N31 at the guide roll 13a, and from there further onto the drying cylinder 10b, proceeding through the group R N31 on support of the same drying wire 12. In addition to the drying module 20A-23 fitted inside the normal group R N31 , it is possible to use drying modules as shown in Figs. 1 to 4 in the group gaps between the normal groups R N1 , R N2 and R N31 . In connection with the drying module fitted inside the normal group R N31 , it is, of course, also possible to employ a closed draw of the web W without an open gap. Fig. 7 shows a dryer section in the other respects similar to that shown in Fig. 6 except that, inside the latter group R N32 , a drying module is fitted that comprises two large cylinders 20A and 20B that have a common drying wire 22. Inside the normal group R N32 , the web W is transferred from the drying cylinder 10a as a short open draw W 0 onto the first large cylinder 20A and from it onto the next drying cylinder 10b, from which the web W is transferred, being guided by the leading cylinder 11a, onto the next drying cylinder 10c. From this drying cylinder 10c the web W is transferred as a short open draw W 0 onto the latter large cylinder 20B, which has a common wire 22 with the preceding large cylinder 20A. After this, the web W is transferred as a short open draw W 0 onto the drying cylinder 10d and further on support of the drying wire 12 of the group R N32 . In the normal group R N32 shown in Fig. 7, it is a difference as compared with a prior-art dryer section provided with twin-wire draw that, after the first large cylinder 20A, the web W runs over two contact-drying cylinders 10b and 10c and over the leading cylinder 11a placed between them before it arrives on the latter large cylinder 20B. It is a further difference that the diameter D 2 of the large cylinders 20A and 20B is larger than the diameter of the contact-drying cylinders 10 and that the large cylinders 20A and 20B are placed at a level lower than the leading cylinders 11. In connection with an embodiment as shown in Fig. 7, it is possible to use a drying module of the sort shown in Figs. 1 to 4 in one or several group gaps between the groups R N1 , R N2 and R N32 . The scope of the present invention also includes such combinations of the embodiments illustrated in Figs. 1 to 4 and in Figs. 6 and 7 in which large cylinders 20,20A are placed both in a group gap and inside a group, each of which large cylinders is provided with a drying wire 22 or its own, or, alternatively, said large cylinders 20,20A are provided with a common drying wire 22 in a way corresponding to the large cylinders 20A and 20B in Fig. 7. In Fig. 8, such a particular dryer section in accordance with the invention is shown in which the main directions T 1 -T 1 , T 2 -T 2 and T 3 -T 3 of the normal groups R NK1 and R NK2 are inclined. In the group gap between the normal groups R NK1 and R NK2 , there is a drying module in accordance with the invention, which module comprises a large cylinder 20, a wire 22 guided by the guide rolls 23 and 23A and arranged so that the web W has a closed draw W 1 to the drying module and a corresponding closed draw W 2 from the wire 22 onto the wire 12 of the group R NK2 . The embodiment shown in Fig. 8 is in the other respects similar to that shown in Fig. 4 except that the plane T 1 -T 1 placed through the centres of the cylinders 10 in the rear end of the preceding normal group R NK1 is inclined downwards and, in a corresponding way, the main direction T 2 -T 2 of the initial part of the latter drying group R NK2 is inclined upwards. The main direction of the rear part of the latter group R NK2 is converted at the leading cylinder 11b into a downwards inclined direction T 3 -T 3 , which is preferably parallel to the plane T 1 -T 1 . The embodiment shown in Fig. 8, and so also the embodiment as shown in Figs. 9 and 10 to be described in the following, has the advantage the length of the dryer section in the machine direction can be reduced further, because an increased drying capacity can be fitted within a metre of length in the machine direction. The embodiment shown in Fig. 9 is in the other respects similar to that shown in Fig. 8 except that the main direction T 1 -T 1 of the rear part of the preceding normal group R NK1 is inclined upwards, and the direction T 2 -T 2 of the initial part of the latter group R NK2 is, in a corresponding way, inclined downwards, the final part of said group being turned in the area of the cylinder 10A into an upwards inclined direction T 3 -T 3 . In Fig. 9, it is a further difference in comparison to Fig. 8 that, in stead of a closed draw, the web W has short open draws W 0 when it arrives on the large cylinder 20 and departs from said cylinder. Fig. 10 shows a dryer section that is in the other respects similar to that shown in Fig. 8 except that the large cylinder 20A is placed inside the normal group R NK , so that the normal group R NK comprises an initial part that is placed before the large cylinder 20A and whose main direction T 1 -T 1 is inclined downwards and a corresponding rear part that is placed after the large cylinder 20A and whose main direction T 2 -T 2 is inclined upwards, so that the web W runs first as an open draw W 0 onto the large cylinder 20A and returns from it likewise as an open draw W 0 onto the same wire 12 in the normal group R NK from which it departed onto the large cylinder 20A. In the other respects, the construction illustrated in Fig. 10 is similar to that described above. The scope of the invention also includes such modifications as shown in Figs. 8 to 10 in which the directions of the planes T 1 -T 1 , T 2 -T 2 and T 3 -T 3 may be even vertical or almost vertical. In the following, the patent claims will be given, and the various details of the invention may show variation within the scope of the inventive idea defined in said claims and differ from those that have been stated above for the sake of example only.

Claims: Dryer section of a paper machine, comprising so-called normal drying groups (R N ) provided with single-wire draw (12), in which the heated contact-drying cylinders (10) are placed in the upper row and the leading cylinders or rolls (11) in the lower row, and in which dryer section there is/are a drying module (20-23) or modules (20A,20B-23)) between and/or inside said normal groups (R N ), in which modules the opposite side of the web (W), in relation to the side placed against said contact-drying cylinders (10), is placed against the heated cylinder face (21), characterized in that said drying module comprises a single drying cylinder (20) or cylinders (20A,20B), against whose said heated cylinder face (21) the web (W) is placed in direct contact, and that said drying module (20;20A,20A,20B,23) comprises a drying-wire loop (22) of its own, which guides the web (W) and presses its opposite side, in relation to the web face placed against the drying cylinders (10) in the normal groups (R N ), against the heated face(s) (21) of said single cylinder (20;20A) or cylinders (20A,20B) over a sector a, whose magnitude has been dimensioned as a > 180°. Dryer section as claimed in claim 1, characterized in that the diameter (D 2 ) or the diameters of said single drying cylinder (20;20A) or cylinders (20A,20B) is/are dimensioned as substantially larger than the diameter (D 0 ) of the contact-drying cylinders (10) in the normal groups (R N ). Dryer section as claimed in claim 1 or 2, characterized in that the dryer section comprises successive normal groups (R N ,R NK ), a drying module (20-23) being fitted in one or several gaps between said normal groups, which drying module preferably comprises a single large cylinder (20). Dryer section as claimed in any of the claims 1 to 3, characterized in that the diameter D 2 or diameters of said single drying cylinder (20;20A) or cylinders (20A,20B) is/are chosen so that D 2 ≈ (1.1...2) x D 0 , preferably D 2 ≈ (1.2...1.7) x D 0 . Dryer section as claimed in any of the claims 1 to 4, characterized in that the dryer section comprises two or more successive normal groups (R N1 ,R N2 , between which there is preferably a closed draw of the web (W), and that said groups are followed by such a normal group or normal groups (R N2 ,R N3 ,R N4 ...) in whose group gaps said drying module is placed that comprises a single drying cylinder (20). Dryer section as claimed in any of the claims 1 to 5, characterized in that, in said single drying cylinder (20;20A) or cylinders (20A,20B), the temperature (t 1 ) of the cylinder face (21) that reaches direct contact with the web (W) to be dried has been chosen as substantially higher than the corresponding surface temperature of the cylinders (10) in the normal groups (R N ), preferably by in the single cylinders employing a higher steam pressure than in the drying cylinders (10) in the normal groups (R N ). Dryer section as claimed in claims 1 to 6, characterized in that the centre of rotation (K) of said single drying cylinder (20) is placed at a level (T 2 -T 2 ) substantially lower than the level (T 1 -T 1 ) of the centres of the leading cylinders (11) or rolls in the normal groups (R N ), and that said difference in height H 1 has been dimensioned in the range of H 1 ≈ 300 mm ... 1500 mm. Dryer section as claimed in any of the claims 1 to 7, characterized in that, in addition to, or in stead of, the drying modules placed in the gaps between the normal groups (R N ), a drying module placed inside the normal groups (R N31 ,R N32 ) is employed, which module comprises a single large cylinder (20A) or several, preferably two, drying cylinders (20A,20B) which have a common drying wire (22) (Figs. 6 and 7). Dryer section as claimed in claim 8, characterized in that, inside the normal group (R N32 ) or groups, two large cylinders (20A,20B) are fitted, which are provided with a common drying wire (22), and that said large cylinders (20A,20B) are placed so that, after the preceding large cylinder (20A), the web (W) is transferred over two contact-drying cylinders (10b,10c) and over the leading cylinder (11a) placed between them onto the latter large cylinder (20B), from which the web (W) is transferred onto the next contact-drying cylinder (10d) in the normal group (R N32 ) and from said cylinder further (Fig. 7). Dryer section as claimed in any of the claims 1 to 9, characterized in that, in the dryer section, in one or several group gaps as well as inside a normal group or groups, a large cylinder (20,20A) is fitted, and that said large cylinders (20,20A) are provided with a drying wire (22) of their own in a group gap and inside a group, or said large cylinders (20,20A) have a common drying wire (22) both in a group gap and inside a group. Dryer section as claimed in any of the claims 1 to 10, characterized in that the upper drying cylinders (10) in successive normal groups (R N ) are placed in the same horizontal plane (Figs. 1 to 6) and/or in inclined different planes (T 1 -T 3 ) (Figs. 9 and 10) and/or in vertical planes. Dryer section as claimed in any of the claims 1 to 11, characterized in that the group-gap draws of the web at said normal groups (R N ) and at the drying cylinder (20;20A) or cylinders (20A,20B) are, at the inlet and/or outlet side of the web (W), closed (W 1 ,W 2 or short open (W 0 ) draws. Dryer section as claimed in any of the claims 1 to 12, characterized in that, in the normal groups (R N ), the lower leading cylinders (11) that are used are leading cylinders (11) provided with a perforated mantle and with an outside grooved face (11'), the inside of said cylinders (11) communicating with a source of negative pressure without an internal suction box, so that in said grooved face (11') such a negative pressure prevails as holds the web (W) reliably on the drying wire (12) while the web (W) is at the side of the outside curve on said leading cylinders (11). Dryer section as claimed in any of the claims 1 to 12, characterized in that the leading cylinders (11) in the normal groups are so-called normal suction rolls provided with a perforated outer mantle and with an internal suction box. Dryer section as claimed in any of the claims 1 to 14, characterized in that, on the single drying cylinder (20,20A) or cylinders (20A,20B), the sector of contact a of the paper web (W) is a ≈ 220° ... 270°. Dryer section as claimed in any of the claims 1 to 15, characterized in that the drying wire (22) of said large cylinder (20;20A) or cylinders (20A,20B) has been tensioned to a tightness (T) that has been chosen higher than the tightnesses of the drying wires (12) in the normal groups (R N ). Dryer section as claimed in claims 1 to 16, characterized in that the horizontal distance (S 2 ) between adjacent drying cylinders (10) in successive normal groups (R N ) is substantially equal to the corresponding horizontal distance inside the normal groups (R N ). Dryer section as claimed in any of the claims 1 to 16, characterized in that the horizontal distance (S 24 ) between adjacent drying cylinders (10) in successive normal groups (R N ) is substantially larger, preferably 2...7 times larger, than the corresponding horizontal distance between adjacent drying cylinders (10) inside the normal groups (R N ). Dryer section as claimed in any of the claims 1 to 18, characterized in that the diameters (D 4 ) of the guide rolls (13A,23A) of the drying wires (12,22) placed in connection with the group-gap draws are substantially larger than the diameters of the other guide rolls (13,23) of said drying wires (12), and that said guide rolls (13A,23A) placed in connection with the group-gap draws are rolls, preferably suction rolls, provided with a grooved and/or perforated mantle (Fig. 4). Dryer section as claimed in any of the claims 1 to 19, characterized in that, inside the group-gap draws and/or the normal groups (R N ), blow boxes (14) are used, by whose means the support contact between the web (W) and the drying wires (12,22) is promoted. Dryer section as claimed in the claims 1 to 20, characterized in that the diameter D 2 or diameters of said single drying cylinder (20;20A) or cylinders (20A,20B) is/are chosen in the range of D 2 ≈ 2000 mm ... 3500 mm, while the diameter D 0 or diameters of the drying cylinders (10) in said normal groups (R N ) have been chosen in the range of D 0 ≈ 1700...2000 mm. Dryer section as claimed in any of the claims 1 to 21, characterized in that, in view of bringing the proportion of the drying taking place on the single drying cylinders (20) to an adequate level, in the normal groups (R) a reduced number of drying cylinders is used, preferably 3...4 drying cylinders (10), and/or that, for the same purpose, the diameter D 0 of the drying cylinders (10) in the normal groups (R N ) is dimensioned smaller than normal, preferably in the range of D 0 ≈ 1500 ... 1750 mm. Dryer section as claimed in any of the claims 1 to 22, characterized in that, in a group gap, the web (W) has closed draws onto the large cylinder (20) and from said cylinder to the following group, and that the wire-guide rolls (13a,23a) placed in connection with said closed draws are solid-mantle, preferably smooth-faced wire-guide rolls with no suction.

Inventors: SIITERI MIKKO, YLI-KAUPPILA JOUKO, SIITERI, MIKKO, YLI-KAUPPILA, JOUKO
Applicants: VALMET CORP, VALMET CORPORATION
Application year: 1994
Main classification: D21F5
IPC codes: D21F5, F26B13